The intraocular calcium activity in the fluid between the retina and pigment epithelium was determined from an analysis of the fast photovoltage responses from the retinal rod cells. It is between 10 to the minus 4th power and 10 to the minus 5th power M, and in darkness this activity causes the rod current of the in vivo retina to be turned on maximally. Light bleaching of rhodopsin appears to result in the formation of two functionally different forms of opsin involved in the control of the light response of the rod cell. One form of opsin has a finite lifetime and is converted into another more stable form that is active in a different way. Calcium and other substances which affect cyclic nucleotide metabolism have been shown to affect the kinetics and sensitivity of retinal rod's response to light. They also have been shown to have pronounced effects on the rate of recovery of the rod current following exposure to bright lights.